1. Field of the Invention
The present invention relates to the field of transducers of the solid state strain gauge type.
2. Art Background
It is well known to fabricate pressure transducers by defining patterns or regions of materials having strain gauges on a flexible diaphragm. In general, commercial embodiments of these transducers are either diffused regions in a monocrystalline diaphragm or a thin film on a metal or other diaphragm.
The solid state semiconductor approach to these transducers (diffused regions in a monocrystalline substrate/diaphragm) has the advantage of using well developed semiconductor processing. This processing includes such steps as growing oxide layers, depositing layers by evaporation sputtering and plating, masking, as well as diffusion, chemical and plasma etching and ion milling. These steps allow the simultaneous fabrication of many identical transducers and compensating circuits, including active networks on the same substrate at a relatively low cost. For an example of such fabrication, see U.S. Pat. Nos. 3,764,950 and 4,033,787. However, these prior art devices have some drawbacks. The current density in the doped regions can be sufficiently high to cause continued diffusion of the dopant into the substrate which changes the resistivity of the gauge. In addition, the deflection of the substrate may cause the relocation of crystal dislocations which may influence carrier mobility, and thereby change the strain gauge resistance. Moreover, mobile ions which pass to the gauge junctions may cause conductivity changes which will also affect stability.
Aside from the above mentioned problems which historically are common in semiconductor strain gauge transducers, the devices possess non-linear thermal sensitivity characteristics when subjected to changes in temperature. In addition, semiconductor transducers will not operate well at temperatures above 125.degree. C. It is commonly required that temperature compensating and calibration resistors be externally coupled to the strain gauge in order to balance the circuit and provide substantially the same electrical output for various temperature environments. The addition of these external resistors results in increased transducer cost. Furthermore, the difference in thermal mass between the resistors and the strain gauge structure and/or the existance of a thermal gradient across the transducer may require a warm-up period of up to several hours before expected precision can be achieved.
As will be described, the present invention provides a semiconductor strain gauge transducer with integral temperature compensating and calibration resistors on the same silicon substrate on which the transducer is formed. Thus, the system of present invention does not require the use of additional substrate or other material, and due to the resistors' location on the same silicon substrate as the transducer, possesses the same warm up characteristics as the strain gauge itself. In addition, the compensating and calibration resistors are formed from the same films which are used to form the strain gauge and adhesion layers as well as the contact pads.